An investigation of the corrosive wear of stainless steels in aqueous slurries

VCEAR w ~ 193 11996)73-7"/

ELSEVIER

A n investigation o f the corrosive wear o f stainless steels in aqueous slurries FanAiming, LongJinming, TaoZiyun Dep~rl~r~r of Malevlals Sciei~c¢ ard Eegineering, Kunming IrtTtlt~te of T~hr~logy. 6JOOgJ K ~ g .

Pg Chi~

R~-eived ~ April 1994;~cepted 3 May 1995

Abstract Pumping installations made of stainleas stee]shave been widely used for wansponing sire'ties in cl~mic~ p~'occss industry. However, knowledge of the attack of stainless steels due to corrosive wear in two-pha~e fiquid-partichi flow is still incomplete. This p~per ~udtes the behaviors and mechanisms of corrosive wear for two austenitic stainless steels, 24Cr-25Ni--4Mo and 18Cr-12Ni-2Mo, using a rotating disc apparatus made by the authors. The two components, wear by slurry abrasion and corrosion, witidn the ~ v e wear ~ ~ first examined individually. Then the synergistic effect belween wear and corrosion is invesogawed. Th~ l ~e.~q~hwe have done shows that co~ro~ive wear rate of samples is closely xelatad to such factors as the solution, type of abrasive, flow velocity, impingemem magle and temprratur¢, in addition, a tl~eshold of flow velocity exists which is called the br~akaway velocity Vk. above which the corrosive weax rate ~ nt pidly. The combined effects of abrasion and corrosion result in a total wear loss larger than the added effects of each lnoce~ alone. An analytical model is developed which would help to reveal Ihe mechanisms of the con'o sire wear processes. K.eyword~: Conv~on wear; Er0¢ior,--cmTosion;Studnless steel

1. I n t r o d u c t i o n In chemical, mining, power and building mate~al iedustales, t l ~ conveying components o f pumps which uansport various slurries fail quickly. This problem has been o f world wide concern and a l o t o f research has been done on this topic [ 1 - 9 ]. The mechanism of corrosive were- and the ayucrgisti¢ effect between COheSion and wear, however, have not been thoroughly dealt with, and many problems still remain unclear. Further stodies m'e needed. In this article, for simplicity, the overall process o f corrosion wear is resolved into two elementary processes, a corrosion process and wear by slurry abrasion. B y investigating their individual behavioPs. the combined process o f corrosion wear can he easily understood, and a mechanism m a y be suggested.

2. E x p e r i m e n t a l m e t h o d s The chemical composition, micresffuctme and hardness o f the stainless steels used are given in Table I. The experiments were conducted with a slurry impeller apparatus designee by the authors, which is shown in Fig. 1. T h e slurry pail with a capacity o f 151 is lined with rubber. The impeller is mode of corrosion w e a r resistant plastic and [X)43-1648196/$15.004D 1996 ~IsevierScieaccS.A. An ~ $~DI 0043-164S C95 ) 066B4-5

leselved

has a diameter o f 2 3 0 r a m whose rotation speed is 1400 rcv rain - i. Specimens a l e 6 t a m thick with a test stirface of 18 mm × 2 5 m m which are mounted on fo,.tr radial positions on the impeller a t three incidence angles with the tangential direction o f the rotation. The slurry used in corrosive wear testing is a m i x t m ¢ of wet phosphoric acid (pHffi l ) and gypsmn ( ~ w - 2 H ~ ) as corrosion m e . u r n end abrasive, respectively. "l'be slurry for pure wear test consists of tap water and gypsum. Both slurries have a mass ratio o f liquid to solid o f 2.5:1. For pure erosion/corrosiou test,only the wet phosphoric acid is used. The wet phosphc~ric acid used in our all tests nmlnly contains following ingredients: P ~ 5 28%, S014- 4 % , F - 2%, C I 200 pprn, Fc 3+ 0.6%. T I ~ par'tide sizv of g y p ~ m n used is 2 0 0 mesh. The test pm'anteters used are given in Table 2. For simplicily, the Ileum" velocity of the moving specimens is approximately mgmded as the flow velocity (relmive velocity between the specimen and the slurry). The erosion test lasted 5 h each time. After o a e test was over, the old slmry was replaced by fresh one for the next test. The mass lOSS o f specimens A W was me.asuxcd using a precLsion a ~ t lyrical balance (Merrier ItS~AR) with a semdlivity o f -t- 0.01 rag. The mass loss rote E was o b ~ u c d with the fc¢-

mula:

74

195 ( 1 ~ )

,4,. leae.AbnlnR e t a L / W e a r

73-77

Table 1

Ck=miceJcomposition, mi¢ro~a'ecmmand hardnessof die specimensused

Chemical composition (wt.%)

Alloy

CA-A?25M lCrl 8Nil2Mo2Ti

Solatimt lemperature ( ~ C )

C

Cr

Ni

Me

Si

Mn

Ti

0.07 0.10

24.1 18.0

25.2 12.6

4.20 2.30

I.L0 1,50

1.50 IfJ0

0.35 0.40

1150 1050

'

Stcactun~

Hatdne.~ ( h'B )

'A+K+X{tr] A+F+K

207 172

A, Austenite;F, Femte; X[~], intermetallic¢omp0UIKI;K, carbide.

Impeltar ~

water slm'ry. The Ec is corrosion rate caused by the Ec is corrosion rate caused by the phosphoric acid.

Spinning oxls

Wo/Irexit~

~turry

Splc~rne~

Slurry pair

oalt

Thlrrnon~eNt

3. E x p e r i m e n t a l results a n d a n a l y s i s

Wafer ~nlroa~

The experimental results exe shown in Fig. 2 and. Fig. 3. From these results, it can be seen that ( 1 ) W h e n the flow velooity Vis below t 2 m s - J, the wear rate E,,, increases slightly with increasing V. W h e n Vexceeds the critical value V,, (here Vk= 12 m s - ]), however, Ew rises drastically, in accordance with a relationship F , ~ = K V ' . Where K i s a constant and n an exponent, the value of which is given in Table 3. A formula exists between corrosion rate E¢ and flow velocityV:

Fig. ]. Rl~s~ation o~IIz¢slurW impeller apparamz. -fable 2

T~t p ~ t e r s

used

Lircar velociw

V(ms-')

Impingement angle l"ew4zemtme

~ (deg) T (°C)

E = A W I S t (g m -2 h -

i5.0 0 40

12.0 30 80

6.0

9.0

90

m)

F~=aV b where S is specimen surface area, r is testing time. I n o u r tests the mass loss rate o f specimens has three notations, E, E w and E c, The E is corrosive wear rate arising from

where a is a constant and b is an exponent relating to the shape o f the specimen (e.g. for a plate b = 0.5). The surface passive layer o f the metals will be more readily removed when flow velocity increases, which consequently

the gypsum-containing phosphoric acid slurry. The Ew is erosive wear rate resulting from the gypsum-containing tap E 8 DO" ~0

:'°° %1

40q

"

I

,°

{~

°°-°

°t

/°'1 o

°

C'

6

9

12

15

3

6,

9

12

15

C

(el V. m / s Fig. 2. The variatima5offrPa-.s$lv*s ot CA-A72SMsteel with flow telocaty Vat diffet~nt impingemem angles at 40 ¢C and 80 °C. E is c~os~ o ~ erosive weaxra~; F~, COtTOSiOatale. The reiatJet, hip between (a) F-~and W.t b ) E, mid It; (¢) E and V. to)

V~ m i t t

tb)

V, m / $

raxe;E~

75

A. FanAindan& e t aL / W e a r J 9 3 (I996) 7 3 - 7 7

Table 3 The magmte~ of the ve~ch-y expommt evaluted by power ~

leads to a significant inct~aee of corrosion rate. Because both E . and E~ d e p e n d o n flow velocity V, the corrosion wear rate E naturally varies also with V. Its c ha ngi ng trend is analogous to that o f E . . ( 2 ) E, E . mad Ec all increase with increasing i mpi nge me nt a n g l e ~x. W h e n a e q u a l s 9 0 ~, the particle jets impinge nor~ m u i l y on the specimen surface at h i g h speed, leading to erosion pitting and carbide spoiling, a n d m a s s loss will be m o r e severe. W h e n a is 0 °, the m o s t obtuse abrasive particles roll a l o n g the sp ecimen surface and only a small amount o f acute panicles, at p r o p e r anterior angle, can lead. to s o m e microc uUings and smalt furrows on the specimen surface.The m a s s loss, therefore, is lower. W h e n ~t is between 0 ° and 9if', m a s s loss o f materials is in the range of the two extreme cases m e n t i o n e d above. ( 3 ) A s the temperature is increased, both E,. and E are enhanced considerably, whereas E~ is "almost unchanged, ff the temperature rises f r o m 4 0 °C to 80 °C, then E¢ increases b y nearly o n e o r d er o f magnitude. T h e reason is that the higher temperature helps to accelerate the electrnchemical reaction, increase the activities of the negative ions F - and C I - in phosphoric acid a n d make the passive layer of the specimen surface less stable, whi c h wiU enhance the corrosion reaction. ( 4 ) T h e w e a r rate Ew is m u c h h i g h e r than the corrosion rate E~ w h e n flow velocity V is above its critical value Vk. Therefore, the w e a r action dominates the corrosion wear process of stainless steels in w e t phosphoric acid slurry, mad the corrosion is a m i n o r factor in the overall w e i g h t c h a n g e recorded. Nevertheless, as the temperature rises the differeac c between the corrosion rate a n d w e a r zata redaees gradually. ( 5 ) U n d e r the s a m e test conditions, the corrosion wear rate o f I Cr I g N i 1 2 M o 2 T i stainl e-s.ssic el i s greater than that of C A -

Alloy

Tf80~

T-40~c

CA-A725M 1CrI SNii2Mo2Ti

~z=0"

a=90"

a=0"

a=90*

2.94 2.72

2.61 2.00

3.07 2.33

2.49 2.Oi

A 7 2 5 M stainless s t e e l T h e reason is that the f r a m e r contains fewer Cr, Hi mad M e elements and h a s a l o w e r hasdness~ s o as to be both less ¢ o r r ~ i o n resistant and less w e a r resistant than the latter.

4. ~ o n

O u r experimental results s h o w that the m a s s loss o f t w o stainless steels in s h m T flow increases w i t h im-ceesiag impact angle. T h e m a x i n m m erosion ¢¢ctws at 9 0 °. T h e relationship is s ome wha t different f r o m what is normally ~ ¢ ~ I for dry solid particle erosion o f dncail¢ ~ u i s . T h i s can b e explaJncM from the ~ o f sbm'y and meal. "nae s l u n i e s consist o f solid imaticles o f g y p s m n in water. C o m p a r e d w i t h other c o m m o n almasives (e.g- q m ) , the gypsuan is very soft ( H V = 7 0 ) .

So its paxticles a ~ e a s i l y b l u n t e d d a r i n g

erosion, which w o u l d lead to a w e a k wear action and a relatively strong impact wear actiom. Moreover, the erosion effect of liquid in a slm3T, generally speaking, is m a i n l y caused b y the normal impact component o f flow velocity. Considering the t w o slurry latices the effect o f impact a a g l e o n wear cma be understood. It w a s ~ | I 0 ] . in fact, that the maxim u m wear rate o f materlals (brittle of ductile) is achieved at

14,

£ 40"C

40at~

90"

"E ~"

8

0 °

b

80 eC

~e

%

g~ x

o"

'

6

(el Fig. 3.11~ v~'iafians of ~

'

9

'

12

6

'

15

V, m / s (bl l ~ s o~ L~rl SNil2Mo2Ti ~

g

9

12

i5

6

9

IZ

t~

v. m/s (c3 V, r o t s with flow ve~oi~ly P at ~ f ~ e ~ em~les.~ oil Y-ui$ ~ f~¢ Fig. 2.

76

A. FanAtmlnB er al. I Wear 193 (1995) 73--77

Table 4 The intentction aad the respective fractions of emsion--corn~ion and wear for twn ~zaiales$steel~ ( ~t: 30~)

Alloy

Row celerity tins -~)

d,E~ (g m -2 h -= )

AERIE (%)

40=~

80~C

40°C

8O~C

40"C

80"C

40~C

E j E {%) S0 °C

1.4 3.6 5.s 5.s

4.7 21.6 30.7 31.3

1.2 3.4 4.2 7.2

5.8 22.4 29.435.1

E~/E ( % )

CA-A725N

15 12 9,0 6.0

4.032 1.060 0.399 0.328

9.989 1.161 0.529 O/4)6

61.9 54.1 45.9 46.6

76.8 4-5 7 38.6 36.9

36.7 42.3 48.3 48.6

lS.S 32.7 30.7 31.8

I CIrL8Nil 2Mo2Ti

1~ 12 9.0 6.0

5.185 1.258 0.544 0.362

9.962 1.428 0.657 0.450

61.4 51.1 46A 45.3

71.5 43.5 37.8 35.2

37.4 45.5 48.3 47.5

22.7 34.1 32.8 29.7

9 0 ° i m p a c t by the erosion o f relatively soft particles. Levy et al. [ 11 ] had found that the wear rate o f soma ductile metals (A1, CU, m i l d steel) increases with a and reaches peak value at normal incidence angle d u r i n g a slurry erosion when the magnitude o f flow velocity is below s o m e values, with which our result is in accordance. The relationship between m a s s loss and impact angle obtained here is also probably related to the property o f the passive film formed o n the surface of a stainless steel in aqueous slurries. T h e passive film is principally a very thin layer of o x i de of C r and Fc (Cr2Os, Fc20 3 ) wh ich is hard a n d brittle. W h e n eroded, the layer would b e cracked and spallad, and is s ubs equc ndy reformed on the fresh metal d u e to sepassivation. T h e c orvoalon wear is a transfer process in which c~rrosive m e d i u m a n d abrasive particles act simultaneously on the material surface. T h e corrosion wear rate o f materials Iargely exceeds the superposition o f the t w o rates at which the material surface is d a m a g e d only by corrosion or only by wear. Apparently, this is the result of the interaction between the corrosion process and the wear process. "].his incremental part o f m a s s loss rate resulting from the interaction o f both is referred to as syucrgisti¢ effact incsement AE=w. So, the corrosion w e a r rate E c a n b e described as E=E,÷E¢+AE~

(1)

T a ble 4 gives the interaction data between corrosion and wear as well as respective relative fractions for stainless steels obtained from experiments. It can b e seen that the synergistic effect i n c s e m e m 5 . E ~ shares a larger fraction in the corrosion wear process compared with the wear c o m p o n e n t and the corrosion component, and heightens w i t h increasing flow velocity a n d temperature. T h e synergistic effect inc,mment A E l , can b e resolved into t w o parls, that is: AE=,=AE=+AE,

(2)

w h e r e A E o tepresenL~ the en ha nc e me nt o f corrosion process caused by wear action, called corrosion increment; A E , represents that o f wear process arising f r o m corrosion action, n a m e d wear increment. T h c u AEc~ is contributed b y these t w o increments. A possible =~¢ol~atiSl=l by whi c h w e a r increases ¢orresion i s as fullow s. T h e untie n of m e e h an ic =11w e a r results in d a m a g e

"

of the passive film leading to exposure o f f ~ h bare surfaces to the eoITosive m e d i u m . ' I b i s accelerates the corrosion process whi c h in turn leads to the d e v e l o p m e n t o f a n e w passive film whi ch will eventually be d a m a g e d b y further mechanical action. ~ t w o subprocesses, wear and passivation, repeat alternately, i f the ability o f the self-passivation o f materials is w e a k and the producing or m e n d i n g o f the passive layer d0¢s n o t catch u p w i t h the d a m a g i n g o f it f r o m mechamioal wear action, than materials will u n d e r g o significant corrosion damage. Another m e c h a n i s m that causes the wear action to enhance corrosion is that the impact o f a h i g h speed liquid- lyarticle b e a m exerted on a material surface can cause the surface to be deformed significantly, hence, to b e i n h o m o g e e e o u s in stress distribution and in electruchemical properties. ,].hose e m b o s s e d parts, such as p l o w ridges, p m t u b e r a u c e s at the pits e dge s and s o on, have high chemical activities a n d can form primary micrucells o f strain difference with adjacent low-strain dom a i n s , so as to accelerate the dissolving process o f anode metals [ 12,13]. F o r reasons g i v e n above, the corrosion increment A E , contains two effects, the o n e that mechanical w e a r action d a m a g e s the superficial passive film AEcr, and the other that the electrochemical properties of~ae surface are v a r i e d by wear action AE==, i.e. AE= ffi AE=~+ A E ~

(3)

A possible m e c h a n i s m w h i c h causes corrosion to reinforce wear m a y be d u e to the following three cases. Firstly, because the corrosion o f a metal is usually n o n u n i f u r m and it star~ at grain boundaries or phase boundaries, corrosion will happen preferentially at the c a r b i d e - m a t r i x interfaces. Accordingly, on the specimen surface e m b o s s e d carbides appear w h i c h are weakly bound with the matrix. ,].hey are broken and spalled very readily under the i m p i n g e m e n t o f abrasive pertleles, s o the carbides scarcely play a role in wear resistance. T h e n , the matrix directly exposed to abrasive s h n r y jet will b e eroded and cut more severely, which re-expoees the carbides. ']'his happens alternately and therefore, wear is enhanced. Secondly, the d a m a g e ofmaterial surface strt~ tare resulting from corrosion can degrade its mechanical properties, s o ehe wear resistance o f materials b e c o m e s poor, w h i c h enhances the

77

A. Fa~qimmg ¢1 a L / Wear 195 (19Y~) 73-7"7

w e a r process [ 14]. Thirdly, because the oxide layer forming on the surface adheres weakly to the base metal, it is easily rubbed away by the s l e n y j e t , tending tO increased wear rates. AS mentioned above, the wear increment A E ~ is composed of three factors, that is: ( l ) the enhancement o f wear from corrosion between pha~es A E w , ( 2 ) the influence of corrosion on mechanical properties o f materlal surface A F-.~, ( 3 ) the enhar~cement of wear from tile oxide layer forming on material surface AE.~, so. AF~=AE~+AF~+

AE,,

(4)

From Eqs. t - 4 : AE

ffiAE~r+,~.E~+AE,.,p+AE,.+AE~

EffiE~+Ew+AEcr+AE~+AE,,p+AE.~+AE.~

2. Corrosion and wear are related t o and ~ by each other. The interaction between both is ~m important factor in the corrosion wear o f stainl&~ steels. 3, The corrosion wear rate o f stainless steels E in~teases with the flow velocity V, impingement angle zEand the medium temperature for ~ conditions studied.

Ae!mowkdgements This research was financially guppofted by the Committee of Science and Technology of Yunnan Province of China.

(5) (6)

Eq. 5 indicates the behavior o f synergistic effect between wear and corrosion, white Eq. 6 gives an analytical expression o f the model for the corrosion wear mechanism of staintess steels. It must be pointed out that the mechanisms suggested for the enhancement o f wear by corrosion and of corrosion by wear are only speculative and should be supported by experimenlal evidence. The authors plan to d o so in further experiments.

~. C o a d u s i o n 1. In w e t phosphoric acid slunT flow. the wear action dominates the corrosion wear and corrosion plays a minor role in the wear process. Nevertheless, the effect of corrosion increases consLderab|y at higher temperature.

References [I]F. Um~mura ~

T- ~

,

Box~

Gijuts~ ~Co~o~cm

[2] A. Hoshlao et ~J~. ~ h o l a t Gijwtsu (ConoMort ~ g i ~ e s ~ g ) , 35 (1986) 212-218 (in 3alpm~ese). [3 ] Y.M. Chang altd C~H.Piu. Co~osi~Y~ '87, P~ver No. 232. [4] J. Postlethwaleet al., CorroMo~. 42 (9) ( 1986) 514--52I. [5t Rao Qichal3g~ d., C/ff~fe J. M'~-t~Fog., 27 (3) ( 1991) l-5 (ill [6] C.H Pin and ¥ . M . C ~ Mbt M*t Prec.,2 (3) (1985) 166. [71 D. Kmly~et al., CorroMon, 44 (4) (1958) 221. [~tl R. 04ua e~ aL, P ~ . 8tk ln~ Conf.. ~ ErosfiTn by Liquid and xoBd Impact. 4--S Sep~'mber1994. C ~ d g ¢ .

UK.

19] B.W. Madsen. Wear, 135 (19~g) 127. [ l 0l W. 2~u ~qd ~ Y . Mao. P~c. i ~ Co~. on Wear ofMateffal& 1987. pp_787-796. t i l l A. Levyand Po Yau. Wear. 98 (19g&) 163. [ [2l R.J. Nael, A. BaH. P¢~¢. Int. C ~ ~ W e ~ ~ Materials, 1983. pp. 14$-152. [13] ]. Hedorrteyer. W e m ' , ~ (19~1) 370-387, [14] Tadashi Usami ~ Yo ~ IMONO. ~0 (197B) 281-286 (ira Japaoese)~